This invention relates to apparatus for the radiometric measurement of the temperature of a target. Particularly, but not exclusively, it relates to radiometric apparatus incorporated in thermal imaging apparatus. The thermal imaging apparatus comprises a scanner for repetitively scanning a thermal radiation detector across the scene in a raster, reconstruction means for producing a visible version of the scene by synchronously scanning a light source across the field of view of an observer in a corresponding raster and modulating the brightness of the source with a signal derived from the detector output.
Thermal imaging devices to which the invention may be applied are described in U.S. Pat. No. 3,626,091 and in published European Patent Application Nos. 0,051,894 and 0,052,395. In these thermal imaging devices, the scanner comprises a polygonal mirror prism having a plurality of planar mirrors arranged at a variety of tilt angles relative to an axis of rotation of the prism. Upon rotation of the prism, the planar mirrors pass through a scanning location where incoming thermal infrared radiation from a remote scene is reflected from each planar mirror in turn to pass through an objective which images the scene upon a linear array of infrared detectors. Each planar mirror of the polygon scans a different band of the scene transversely across the length of the array. Each detector is coupled to modulate one of a corresponding linear array of light sources. The light sources are viewed by reflection in the planar mirrors as they pass through a reconstruction location separated from the scanning location around the prism. The sequence of mirror tilt angles around the prism is chosen in relation to the separation of the scanning and reconstruction locations around the prism so that a correctly reconstructed image is presented to an observer.
British Patent Specification No. 1,442,195 describes a thermal imaging system in which the visible picture is reconstructed on the face of a cathode ray tube. In addition to modulating the electron beam of the cathode ray tube, the output of the single infrared detector is digitized at intervals in the raster and stored in a digital memory for comparison with the corresponding values of a laser raster. If the difference in detector outputs at corresponding points in the two rasters exceeds a threshold value corresponding to an undesirably rapid change in temperature at that point in the scene, an alarm is raised. There is, however, no measurement of the temperature at any point in the scene.
Radiation pyrometers for measuring of the temperatures of remote targets are known in which a controllable internal radiation reference source is used. Radiation from the reference source is compared with radiation from the target by an infrared detector by means of a chopper. The chopper switches the field of view of the detector alternately between the target and the reference source. The detector output is then a difference signal which, via a phase sensitive rectifier and an amplifier, is used to control the radiation output of the reference source so that the radiation received from the reference source and the target are equal. The temperature of the reference souce, or alternatively the power energizing the reference source, may then be taken as a measure of target temperature, due allowance being made for the emissivity of the target. Such a radiometer is described in Mullard Technical Note No. 79 (TP 1664) published in March of 1978.